1. Field of the Invention
The present invention relates to a method for obtaining thermoplastic resin films of good offset printability and good melt thermal transfer printability capable of giving prints of good water resistance, which comprises the steps of subjecting the surface of the thermoplastic resin film to specific oxidation, then applying a specific surface modifier thereto to form a layer on it, and thereafter stretching the resultant in at least one direction.
2. Description of the Background
Coated paper is heretofore used for the base of outdoor propaganda stickers as well as that of adhesive labels to be stuck to frozen food containers, but its water resistance is unsatisfactory, and therefore it is reinforced with a polyester film applied on the surface thereof.
Recently, thermoplastic resin film of good water resistance, especially polyolefin-based synthetic paper has become much noticed as a hopeful material capable of being substituted for the polyester film-coated paper for the base of such labels. The resin film of the type is known, and its details are described in, for example, Japanese Examined Patent Publication (KOKOKU) No. 46-40794 and 49-1782, and Japanese Unexamined Patent Publication (KOKAI) No. 56-118437, 57-12642 and 57-56224.
However, since its material, polyolefin, is non-polar, the polyolefin-based synthetic paper is not always satisfactory in point of its printability in offset printing, gravure printing, relief printing, flexographic printing or the like and in point of its compatibility with melt thermal transfer printers, sublimation thermal transfer printers and others. Therefore, in general, it is subjected to suitable surface treatment before use.
One embodiment of such surface treatment known in the art is a method that comprises the steps of subjecting the surface of a non-stretched film for production of polyolefin synthetic paper to corona discharge treatment for oxidation, then applying a coating liquid to its surface, and then stretching the film, and optionally further subjecting the surface of the film to oxidation such as corona discharge treatment. For example, Japanese Unexamined Patent Publication (KOKAI) No. 7-266417 discloses a method for producing a laminate resin film that comprises the steps of subjecting a longitudinally-stretched film to corona treatment at 30 to 100 W·min/m2, then coating the processed surface of the film with a coating liquid of ethylene-vinyl alcohol copolymer having an ethylene content of from 20 to 45 mol %, drying it, then stretching it in the lateral direction by the use of a tenter, and further subjecting it to corona treatment at 30 to 100 W·min/m2. However, the laminate resin film produced according to the method is unsatisfactory in point of its melt thermal transfer printability and offset printability, and therefore, improving it in that point is desired.
Japanese Unexamined Patent Publication (KOKAI) No. 11-323267 discloses a film of good relief printability and good adhesiveness, which is fabricated by forming, on a polymer film, an adhesive layer that contains a copolymer of ethylene and an ethylenic unsaturated compound having an adhesiveness-imparting functional group wherein the ethylene content in the copolymer falls within a specific range. However, the adhesive film is also unsatisfactory in point of its offset printability and melt thermal transfer printability, and improving it is desired.
We, the present inventors, have proposed an image-receiving film for thermal transfer, which is fabricated by applying an aqueous resin dispersion to a thermoplastic resin film followed by drying it thereon and for which the resin dispersion is prepared by dispersing an olefin copolymer (a) with an unsaturated carboxylic acid or its anhydride bonded thereto in water along with at least one dispersant (b) selected from the group consisting of nonionic surfactants, nonionic water-soluble polymers, cationic surfactants and cationic water-soluble polymers, the ratio by weight of (a)/(b) in terms of the solid content thereof falls between 100/1 and 100/30 and the mean particle size of the dispersion is at most 5 μm (Japanese Patent Application No. 2000-365265).
Though its melt thermal transferability is good, the image-receiving film for thermal transfer is problematic in its offset printability, especially in point of the water resistance of the offset prints given by it, and there is still room for improvement in the film.
An object of the present invention is to provide a thermoplastic resin film of good melt thermal transfer printability and good offset printability capable of giving prints of good water resistance.